1. Field of the Invention
This invention relates generally semiconductor fabrication techniques. More particularly, but not by way of limitation, this invention relates to tools, methods, and devices for mitigation of extreme ultraviolet (EUV) optics contamination—the EUV optics that may be used in EUV lithography applications.
2. Description of the Related Art
Lithography is a process commonly used in semiconductor fabrication. Generally, lithography requires the use of light radiation to transfer a pattern on to the surface of a substrate. The particular surface may include a light-sensitive chemical (e.g., photoresist), and through one or more chemical treatments the pattern can be transferred, deposited, and/or etched on to the substrate.
EUV lithography uses extreme ultraviolet radiation as a light source. EUV light, having a wavelength of approximately 13.5 nm (e.g., in the range of 13-14 nm), may be absorbed by all types of matter, and as such, the optics used to transfer the EUV light on the surface of the semiconductor substrate may be contaminated through carbonization and/or oxidation. Moreover, in the presence of EUV light radiation, carbonization and/or oxidation reactions may be initiated on the surface of the EUV optic by either photons, electrons, holes, and/or excitons.
For example, FIG. 1 illustrates the potential contamination of EUV optic 103 in more detail. As shown, light source 101 produces EUV light radiation and photons 102. As depicted by the zig-zag line, these photons 102 may be absorbed into the surface of EUV optic 103. As described by the photoelectric effect, the photoabsorbed photons may produce one or more electrons/holes 104. The generated electrons/holes may either (1) not reach the surface at all (depending on the energy of the electron/hole and the distance to the surface), (2) reach the surface without scattering, (3), reach the surface after elastic scattering, and/or (4) reach the surface after inelastic scattering. Particularly with respect to inelastic scattering, the generated electrons/holes (e.g., primary electrons) may lose energy and generate another electron (e.g., secondary electrons) which may be ejected from the atom where the scattering takes pace. The incoming photons, having a kinetic energy of approximately 93 eV, may generate primary electrons/holes having a maximal kinetic energy of 90 eV (i.e., the 92 eV associated with the incoming photon minus the binding energy of a valence electron). The majority of electrons/holes that reach the optics surface, however, have kinetic energies below 20 eV.
The photons and electrons/holes produced by these photon induced reactions and secondary electron induced reactions may interact with the molecules on and around the surface of the EUV optic 103. Specifically, a carbonization reaction with hydrocarbons 106 can react to produce physically adsorbed hydrocarbons 108. These physically adsorbed hydrocarbons 108 can subsequently produce contaminant layer 105 (e.g., a growing carbonaceous film). Additionally, an oxidation reaction with residual water molecules 107 may also contaminate the EUV optics and add to contaminant layer 105.
Several solutions have been suggested to reduce the contamination of the EUV optics created by the EUV light radiation. These solutions, however, may only partially solve the contamination problem or may be very costly. For example some have suggested cleaner practice. EUV lithography is generally performed in situ (e.g., in a vacuum), and some contamination may be reduced simply by reducing the residual water vapor and hydrocarbons present. No solution has been presented that can eliminate the residual water vapor and/or hydrocarbons, and thus, such a solution has proved lacking.
Several solutions propose cleaning the EUV optics. For example, an in-situ and/or ex-situ etching processes have been proposed. See e.g., Gubbini et al., “‘On-line’ cleaning of optical components in a multi TW-Ti:Sa laser system,” Vacuum, 76, 45 (2004); Klebanoff et al., “Method for in-situ cleaning of carbon contaminated surface,” U.S. Pat. No. 7,147,722 (2006); Kakutani et al, “Carbon Deposition in multi-layer mirrors by extreme ultraviolet radiation,” Proc. SPIE, 6517, 651731 (2007); Nii et al., “Performance of Cr mask for extreme ultraviolet lithography,” Proc. SPIE, 4409, 0227-786X (2001). Additionally, an in-situ/ex-situ atomic hydrogen and/or molecular oxygen cleaning solution has been proposed. See e.g., Graham et al., “Atomic hydrogen cleaning of EUV multilayer optics,” Proc. SPIE, 5037, 460 (2003). Also, an ex-situ laser exposure solution has been proposed. See e.g., Tanaka et al., “Cleaning Characteristics of Contaminated Imaging Optics Using 172 nm Radiation,” Jpn. J. Appl. Phys., 46, 6150 (2007). These cleaning solutions may have unintended side-effects. For example, these solutions may actually enhance the oxidation of residual water vapor. The cleaning methods as referenced above can affect the capping layer properties of an EUV optic—the use of a capping layer is discussed in more detail below. The degradation of the EUV capping layer due to the cleaning may decrease the reflectivity of the multilayers with time and thus reduce the lifetime of EUV optics. Furthermore, these cleaning solutions may require the removal of one or more EUV optics from the system so that they may be cleaned. Such a requirement could considerably increase time and cost.
Other solutions propose the use and/or modification of a capping layer to be used on the surface of the EUV optics. See e.g., Sa{hacek over (s)}a Bajt et al., “Oxidation resistance and microstructure of ruthenium-capped extreme ultraviolet lithography multilayers”, J. Microlith., Microfab., Microsyst., 5, 023004 (2006). Ru is currently accepted and utilized as industry wide capping layer for EUV optic. The oxidation of Ru capped EUV optic is decreased but carbon contamination is still an existing problem for Ru capped EUV optic. Carbon growth on Ru capped EUV optic is widely reported in literature however oxidation can also be seen on Ru capped EUV optic but at slower rates.